An AC electric vehicle equipped with a converter and an inverter uses the converter to convert high voltage AC electric power from an AC power supply into DC electric power, uses the inverter for conversion into variable voltage and variable frequency 3-phase AC electric power, and drives the electric motor. In order to maintain a stable output DC voltage of the converter, the DC current side of the converter is connected to a filter capacitor in parallel with the load (inverter). The converter and inverter can perform regenerative braking. During regenerative braking, in a manner opposite to the aforementioned operation, AC electric power generated by the electric motor is returned from a pantograph to the AC power supply.
However, the regenerative load as seen by the electric vehicle during regenerative braking may be reduced suddenly due to pantograph-overhead line disconnection, rapid change of load of another electric vehicle connected to the overhead line and the like, and voltage across the filter capacitor may become higher than normal. When voltage across the capacitor becomes high, an abnormal voltage is applied to the semiconductor elements of the converter and inverter, and to the filter capacitor. Application of abnormal voltage damages these components, and thus to avoid such damage, an overvoltage suppression circuit is arranged to form a short circuit across the filter capacitor through a resistance.
That is to say, when voltage across the filter capacitor is detected to have become higher than a predetermined value, the gates of the inverter and converter elements are turned OFF, and the regenerative braking operation stops. Then the overvoltage suppression switch closes so that the filter capacitor is shorted through the overvoltage suppression resistor, the regeneration electric power output by the inverter is released, and thus voltage of the filter capacitor is lowered. If this condition continues unchecked, current continues to flow in the overvoltage suppression resistor from the AC power supply side through the diodes of the converter. Thus the converter input-side AC switch is opened simultaneous with the closing of the overvoltage suppression switch.
Patent Literature 1 describes a power conversion device mounted in an AC electric vehicle, in which the power conversion device avoids overheating damage of the overvoltage suppression resistor, even when the normal release operation does not occur, for example, due to failure of the AC switch and the like. According to the power conversion device of Patent Literature 1, a current detector is provided that detects current in the overvoltage suppression resistor, and if the AC switch does not open normally, an AC circuit breaker is opened according to the condition of output from the current detector, and current from the AC power supply side to the overvoltage suppression resistor is cut off.
Patent Literature 2 mentions charging and discharging of a filter capacitor, improvement of utilization rate of initial charge resistance and a switch for initial charging, and preparation for the occurrence of erroneous simultaneous closing of the switch for initial charging and a switch for discharging. A discharge circuit of a main circuit capacitor of the Patent Literature 2 is equipped with a discharging switch in a regeneration function-equipped sine wave converter which supplies electric power of an AC power supply through an AC reactor and a closed circuit to an initial charging circuit, a three-phase bridge circuit and a main capacitor that are inserted in the circuit. By closing the discharging switch, a circuit is formed for using the initial charge resistance forming the initial charging circuit as a discharging resistance.
In addition, Patent Literature 3 describes closing a contact for charging, and firing a self-arc extinguishing element of a converter to discharge a filter capacitor (FIG. of Patent Literature 3). Moreover, Patent Literature 4 describes current flowing through a circuit that includes a converter element, an initial charging contact and a charging resistance thereof, to cause discharge of a capacitor (FIG. 5 of Patent Literature 4).